Service providers typically offer numerous voice and data services to end users. Some examples of voice services are voice calls, call forwarding, call waiting, etc. Some examples of data services are Internet access, streaming audio, streaming video, online gaming, IP-TV, etc.
The first types of wireless or mobile networks that were introduced by services providers were First Generation (1G) and Second Generation (2G) networks. 1G networks provided voice services via analog signals, and then evolved into 2G networks that provided voice services via digital signals. Mobile communications then evolved into 3G (including 2.5G) networks that provided both voice services and data services. For example, 3G networks are able to provide wireless voice telephony, as well as data services such as Internet access, video calls, mobile TV, etc. Some of the 3G networks implemented by service providers were Universal Mobile Telecommunications System (UMTS) networks, EV-DO networks, General Packet Radio Service (GPRS) networks, etc. Service providers are now beginning to migrate their networks toward Fourth Generation (4G) technologies. 4G networks are essentially enhancements to 3G networks in terms of data speeds. For example, a 3G network can provide data speeds of about 3.5 Mbit/sec. According to the International Telecommunication Union (ITU), a 4G network can provide data speeds of 100 Mbit/sec. One example of a 4G network is a Long Term Evolution (LTE) network.
Presently, 4G communications are available in limited areas, such as large cities. When a mobile device is in the coverage area of a 4G network, the mobile device is able to attach to the 4G network to receive services. When a 4G network is not available, the mobile device may be in the coverage area of a 3G (or 2G) network in many cases. Thus, the mobile device can register with the 3G network to receive services. Mobile devices that are able to register with both 4G and 3G networks may be referred to as dual mode or multi-mode devices.
Early implementations of 4G networks provide data services but do not provide voice services for mobile devices (assuming VoIP services are not available through the IP Multimedia Subsystem (IMS)). Therefore, in order for the mobile device to place or receive voice calls, the mobile device needs to also attach to a Circuit-Switched (CS) core network (2G or 3G) to access voice services. The CS core network that provides voice services is referred to as the CS domain, while the data portion of a 3G or 4G network that provides data services is referred to as the Packet-Switched (PS) domain. When a mobile device switches from a 4G network to a CS core network to originate or terminate voice calls, this is referred to as “CS Fallback”.
In order to serve a mobile device, the CS domain and the PS domain perform mobility management procedures such as attach, paging, and location update procedures. The attach procedure is a process that a mobile device performs to register with a network before the mobile device can access services. The paging procedure is a process where the network contacts the mobile device using control messages. Location update is a procedure where the location of a mobile device is reported to the network.
Both the CS domain and the PS domain store a Mobility Management (MM) context for a mobile device, which indicates the mobility information for the device. For example, the MM context for a mobile device may include an International Mobile Subscriber Identity (IMSI), a MM state, a Temporary Mobile Subscriber Identity (TMSI or P-TMSI), a location area or routing area, a Mobile Station ISDN Number (MSISDN), ciphering algorithms, etc. Most networks allow a mobile device to simultaneously connect to the CS domain and the PS domain, which is sometimes referred to as a Class A operation mode or PS/CS operation mode. When a mobile device is allowed to simultaneously attach to the CS domain and the PS domain, MM data can become desynchronized across the CS and PS domains. If MM data is not synchronized, mobility management procedures may not be performed correctly. For example, paging procedures to the mobile device may not be performed correctly if the MM data is not synchronized between the CS and PS domains.